1. Field of the Invention
The present invention relates to a semiconductor device with a semi-metal having superior characteristics such as a high switching speed, high current gain in the microwave region, and so forth.
2. Description of the Related Art
In general, electrons having a high energy, e.g., an energy level higher than the Fermi energy by 2 to 3 kT (k: Boltsman constant, T: lattice temperature), are referred to as hot electrons. Several semiconductor devices are known in which hot electrons are used for transportation between one terminal and the other terminal in a three-terminal transistor similar to the bipolar transistor.
In such semiconductor devices, only the metal base transistor (MBT) (see, for example, S. M. Sze and H. K. Gummel "Appraisal of Semiconductor-Metal-Semiconductor Transistors," Solid State Electronics, vol. 9, p. 751 (1966)) having superior microwave characteristics with a high current gain is known.
In the MBT, however, since the Schottky barrier between the semiconductor emitter or collector and the metal base is too high, the reflection factor of the carriers at the interface between the semiconductor layer and the metal layer is large. Therefore, the injection efficiency from the emitter to the base is low.
Further, in the metal base layer, the injected carriers are scattered by the electrons so that a considerable amount of energy of the carriers is lost in the base layer. This result in a large base current and small current amplification factor.
In a semiconductor device in which the above-mentioned first drawback of the MBT is eliminated, a hot-electron transistor (HET) is known "Tunneling Hot Electron Transfer Amplifiers (THETA): A Proposal for Novel Amplifiers Operating in the Subpicosecond Range", M. Heiblum, International Electron Device Meeting, p. 629 (1980).
In the HET, a high density n-type semiconductor is used as base material so that the potential barrier between the emitter or the collector and the base is smaller than that in the MBT. The HET, however, has a drawback of a larger base resistance than the MBT, which causes a lowering of the maximum oscillating frequency. To lower the maximum oscillating frequency in the HET, the base length can be elongated. The long base length, however, also lowers the maximum oscillating frequency, and therefore, it is not preferable in the HET to elongate the base length to lower the base resistance.
Apart from the above-mentioned MBT or HET, Japanese Unexamined Patent Publication No. 62-45064, published on Feb. 27, 1987, discloses a four-terminal heterojunction semiconductor device of. In this device, a semi-metal is realized only when a bias voltage is applied to the device.
Further, a semiconductor-semimetal transistor in InAs-GaSb superlattices is mentioned in a paper of Appl. Phys. Lett. 35 (12), 15 December, 1979, 0003-6951/79/240939, pp. 939-942. In this paper, it is disclosed that a semi-metal state is realized in a GaSb-InAs superlattice when the InAs layer thickness is between 100 .ANG. and 200 .ANG..